Stress relieved grains

ABSTRACT

An internally ported grain having an active solids loading of from 85 to 98 percent. The configuration of the port is determined so as to provide a desired internal burning surface and relief for stored strain energy.

United States Patent Jordan et a1.

[54] STRESS RELIEVED GRAINS [72] Inventors: Frank W. Jordan, McGregor;Leonard D. Webb, College Station,

[151 3,691,955 1 Sept. 19, 1972 2,750,887 6/1956 Marcus ..60/35.6 RS

3,033,117 5/1962 Bonner ..60/253 3,090,196 5/1963 Brewer ..60l2553,193,883 7/1965 Thibodaux et a1. 102/99 X OTHER PUBLICATIONS Vogel, JetPropulsion, Feb. 1956, pages 102- 105 Stone, Jet Propulsion, April 1956,pages 236- 244 Primary Examiner-Robert F. Stahl Attorney-William R.Lane, Thomas S. MacDonald and Richard L. Mikesell 2 5 7] ABSTRACT Aninternally ported grain having an active solids loading of from 85 to 98percent. The configuration of the port is determined so as to provide adesired internal burning surface and relief for stored strain energy.

2 Claims, 11 Drawing Figures PATENTED EP 3.691.955

sum 2 or 2 I N VENTORS.

ZEO/V/IAD 0- W555 FRANK 14 JORDAN Mm/M STRESS RELIEVED GRAINS PRIOR ARTThe oldest solid propellant grain design is a simple solid cylinderwhich is burned from the aft end of the missile towards the forward end.This type of grain is termed an end burning grain and is frequentlysuitable for very small charges of propellants that burn at lowtemperatures. However, for larger charges or propellants having a higherburning temperature, it is necessary to insulate the wall of the motorfrom the flame. The coating of the insulative surface reduces the amountof propellant which can be placed in the motor, thereby reducing thevolumetric loading. For the purpose of this disclosure, volumetricloading is defined as the volume of the active solids within the motordivided by the volume inside the motor casing. Active solids, of course,are those which contribute directly to the impulse of the motor.

To eliminate the need for internal insulation, internal burningpropellant charges have been used. These charges have a port or portsrunning axially through the grain such that the entire length of thegrain burns simultaneously and the flame travels from the center of thegrain out towards the wall.

In such internal burning grains, the propellant serves to protect theouter case from the hot gases that evolve. Additionally, one is betterable to program the burning rate and thrust to be derived from the motorthrough the use of various design configurations of the internal port.One of the most prevalent examples is the star center propellant chargehaving the internal port in a general configuration of a multipointedstar. In internally ported grains two significant problems arise. First,the volumetric loading of a given motor casing is affected by the portarea. In other words, 100 percent volumetric loading cannot be achievedutilizing normally internally ported grains. A typical motor only hasfrom 70 to 80 percent volumetric loading. Second, there is a tendency ofthe grain to crack at stress concentration points about the port area.Any such cracking detrimentally affects the performance of the grain andoften leads to a catastrophic failure of the entire motor.

An object of this invention is to eliminate grain cracks in internallyported grains.

A further object of this invention is to provide motors having very highvolumetric loading.

Further objects of this invention will become apparent from thefollowing description.

SUMMARY OF THE INVENTION The above and other objects of this inventionare ac complished by cutting the propellant grain with a thin knifeblade in a configuration to expose the desired surface area while takingadvantage of the relief in the grain stresses that such a controlled outcan accomplish. Thus, an internal cut in the grain controls both strainenergy buildup as well as defining the initial burning surfaces. Thecuts are made such that in one dimension normal to the axis of thegrain, the width of the cut is negligible. The width of the cuts are sosmall that the volume loading of the motor is from 85 to 98 percent.

However, the slots of the instant invention must be of sufficient widthto properly vent combustion gases through the throat of the motor. Ifthe venting area is insufficient, pressure build-up within the motor cancause catastrophic failure. It can be seen that the range of widths ofthe slots of the instant invention is a function of, e.g., propellantcombustion and throat area. However, in most propellant compositions andtypical throat areas, cuts ranging in widths from 0.5 to 5 percent ofthe diameter of the grain are suitable.

BRIEF DESCRIPTION OF THE DRAWINGS It is believed that the invention willbe better understood from the following detailed description anddrawings in which FIGS. 1a 1c are cross-sectional views of propellantgrains provided with the internal cuts of this invention. 7

FIGS. 2a 2c are respectively side views of the grains of FIG. la 1c.

FIGS. 3a 3c depict cutting means to provide the cuts respectively onFIGS. 1a 10.

FIG. 4 discloses a section of a grain cut having a mandrel disposedtherein.

FIG. 5 is the same grain cut as FIG. 4; however, the propellantsurrounding the cut has been cured whereby the cut is opened due to theshrinkage of the propellant.

PRINCIPLE OF THE INVENTION The principle of this invention utilizing anextremely thin cut in a solid propellant grain uniquely combinesprinciples of internal ballistics and grain stress relief. As will beexplained, two internal ballistic phenomena are combined through thisinvention to produce an effect which is contrary to the effect producedby either of the phenomena separately. These two ballistic phenomenainvolved are (l) erosive burning and (2) flame propogation rate. Inconventional solid propellant rocket motors with internal burninggrains, erosive burning generally has a detrimental effect and is notdesirable. When the motor is initially ignited the port cross-sectionalarea is the smallest. If the throat area of the nozzle of the motor ishalf as large as the port area of the motor, the Mach number of theexhaust gases at the nozzle end of the grain is usually high. These highvelocity gases sweep the surface of the grain, eroding the softenedpropellant from the grain prior to its being efiectively burned. Theresult is a highly increased effective burning rate and very highchamber pressures. In order to reduce the effect of erosive burning, theinitial port area has to be increased or the initial burning surfacedecreased. Typically the erosive burning rate may be four times theregular burning rate of a motor for composite solid propellants.

The second internal ballistic phenomena, flame propogation rate, can beeither harmful or helpful to the motor performance. If the flamepropogation is used to aid motor ignition, then it can be helpful.Alternatively, when the flame propogation occurs in an unplanned graincrack, it is apparent that the uncontrolled burning of the grain in thismanner would be deleterious. Flame propogation rate is very high whentraveling in the direction of the combustion gases. In this direction itis on the order of a thousand inches per second. On the other hand, theflame propogation rate is relatively low, only about 200 inches persecond, when traveling in the opposite direction. The motor of thisinvention combines the two ballistic phenomena in such a way as topermit the increase of volumetric loading to nearly 100 percent. Themotor, as indicated, having a knife blade width slot cut therein, isignited at the nozzle end and the flame propogates along the slit fromthe aft end forward. Due to the very small port area, the erosion on theaft end of the grain is very high but the burning surface is very low.As the flame travels to the forward end of the motor increasing theburning surface, the aft end erosion opens the port. As the port opens,the erosion decreases.

Having now explained the theory of the burning along the knife bladecut, attention should be directed to the grain stress relief provided bythe cut. Two problems confronting the designers of solid propellantgrains are intentional or unintentional debonding between the case andthe propellant grain, and cracking of the inner bore. Past attempts toincrease volumetric loading in case bonded systems resulted in severegrain stress and strain states. Ballisticians are limited in theirdesign of port configurations because of the problem of inner borecracking. The configurations that eliminate the possibility of crackingare seldom compatible with the demand for high volumetric loading. Theconfiguration of the internal cut in the grain can be optimized toprevent unwanted cracking using the Griffith energy hypothesis. In thepaper The Role of Fracture Mechanics in the Design of Optimum Grain-CaseTerminations by .l. S. Noel and L. D. Webb, CPIA Publication No. ll9,Vol. 1, Oct. 1966 (available from the Chemical Propulsion InformationAgency, John Hopkins University, Applied Physics Laboratory, 8621Georgia Avenue, Silver Spring, Maryland), the authors discussed theseenergy balance concepts as applied to end releases in solid propellantgrains. One can utilize the information in the paper to apply these sameconcepts to optimize the location of the cuts made according to thisinvention. In addition, the potentially destructive energy induced andstored within the grain can be reduced to a minimum by proper locationof grain stress relief systems. These relief systems consist, as in thepresent invention, of intentional internal grain cuts which not onlycontrol strain energy buildup but also define initial burning surfaces.The grain design that results has very high volume loading with highstructural integrity while pos sessing a port area of nearly zero. Asexplained, the flame propogation rate down the crack could be utilizedalong with erosive burning effects to ignite and open the port for aninternal burning configuration. Grain stress considerations and desiredport configurations will control the cross-sectional shapes of theburning surfaces.

Turning now to FIGS. la 1c, there is shown a sectional view ofpropellant grains having various designed internal cuts. It has beenfound that particular designs are suitable for both stress relief andprevention of continued crack growth. For example, turning to FIG. la,there is shown an S configuration 11 cut within a propellant grain 13.As shown the grain is bonded to a case 15. Both ends of the S 1'17 and19 respectively, are curved. It is generally found that curvatures atthe end of such cracks serve to prevent further growth of the cut, or inother words, propogation of the crack. The exact terminus of the end 17and 19 can be determined from calculations as disclosed in the articleby Noel and Webb such that they are located in areas of minimum stressin the grain. As can be appreciated, utilizing the concepts disclosed inthe article, one can predict the stresses at any given point within thegrain structure. It should be apparent that it is most desirable toplace the cut 11 along a line defining areas of maximum internal stress,whereby the cut can serve as a stress relief means. The terminal ends ofthe out should be placed in areas of minimum stress so as to preventfurther propogation thereof prior to and during burning of the grain.Fig. 1b discloses a variation having three cuts 21, 23 and 25 emanatingapart and extending radially from a central point 27. Each of the cutsis terminated by a perpendicular short line such as 29 perpendicularthereto that is curved at its end inwardly toward the center point 27.As can be seen the general configuration appears as three T shaped cutsemanating from the central point 27. However as can be seen rather thana plain T, the ends of the top of the Ts are curved inwardly so as toprevent as previously indicated further extension of the cut duringgrain storage, and prior to firing. FIG. 10 is a variation of theconfiguration of 1b wherein three curved lines 31 emanate from a centralpoint 33. The possible configuration of cuts is innumerable and greatlydepends upon the design of the motor as well as the characteristics ofthe propellant charge itself. FIG. 2a illustrates the cross-sectionalside view along the entire length of propellant grain 13 displaying thecut 1 1 partially shown.

Turning now to FIGS. 30 3c, there are shown devices used to cut theshapes respectively shown in FIGS. la 1c. The device used can be likenedto a cookie cutter, in that thin sheet metal as shown in FIG. 3a isformed to the configuration 35 of the S cut in grain shown in FIG. 1a.Attached to the sheet metal cutter 35 is a rod 37 which would beequivalent to the length of the propellant grain. Prior to casting ofthe propellant, the rod and cutter is inserted into the case with thecutter being at the head end of the propellant. After curing, the rod isthen pulled out of the grain from the rearward end thereof. Thus, thecutter 35 will traverse the entire grain cutting the desiredconfiguration thereon. Alternative to casting the cutter in place andwithdrawing it after the grain is cured, it is possible to first cure apropellant grain and then push the cutter through the cured grain fromone end to another where both ends are exposed prior to their enclosureby bulkhead and nozzle attachments.

Turning now to FIG. 4, there is shown a portion of a I I cut 39surrounding a thin metal mandrel 41. In this instance, the mandrel 41will extend the entire length of the grain and is placed in the castingmold prior to the propellant. The drawing is enlarged and shows spacingbetween the edge of the cut 39 and the mandrel 41. However, as can beappreciated, in practical application the propellant materialsurrounding the cut is actually in contact with the mandrel material.The relationship shown in FIG. 4 is in the precured condition. However,once the propellant has been cured, there is significant shrinkage suchthat the cuts surrounding the mandrel are slightly open. The shrinkageis in the order of 0.6 percent for composite solid propellants. Theresultant opening is shown in FIG. 5 and facilitates the easy removal ofthe thin metal mandrel from the entire grain leaving a slight opening orcrack 43 as shown in H6. 5. a

The concept of the invention is applicable to any type of solidpropellant including composite and double based ones whenever aninternal port is desired. Following is an example of the predictedefiect of the utilization of this invention. Simplified calculations areused, as the purpose is merely to illustrate the advantages of theinvention. Visualize a solid propellant composition of carboxyterminated linear polybutadiene as a binder having solid particleammonium perchlorate therein as an oxidizer and aluminum powder as afuel. A typical propogation rate would be 200 inches per second. Theerosive burning rate of such a composition, for example, could be 2inches per second. Assuming a motor length of 50 inches with a singleslot, the flame front would travel from the nozzle to the forward end ina travel time of approximately 0.25 second. By that time, the aft endslot width is (2 inches per sec) (.25 sec) (2) 1.0 inches. If a singleslot 5 inches wide is used, and the burning produces a wedge shapedvolume, the volume would be about (5 inches)(! inch)(25 inches)/2 62.5cubic inches. If the starting volume of the slot is negligible, 62.5cubic inches of propellant have been burned by the time the flamereaches the forward end of the grain. In prior art grains, this 62.5cubic inches of propellant could not have been included in an internallyported grain. This additional propellant therefore represents asignificant increase in total impulse over prior art motors.

We claim:

1. A stress relieved internal burning solid rocket propellant grainhaving an active solids content of from to 98 percent by volumecharacterized by a series of intersecting slits located generally alonga line parallel to the axis of the grain and having a width of between0.5 and 5 percent of the diameter of the grain, at least one slitpassing through the central axis of the grain and having other slitscurved in cross-sectional shape located between said central axis andthe outer periphery of the grain.

2. A stress relieved "internally, burning solid rocket propellant grainhaving an active solids content of from 85 to 98 percent by volumecharacterized by at least one slit extending lengthwise along the entiregrain and located generally along a line parallel to the axis of thegrain in an area of said grain having maximum internal stress and havinga width of between 0.5 and 5 percent of the diameter of the grain, saidslit having end portions curved in cross-section terminating in areas ofthe grain having minimum internal stress.

1. A stress relieved internal burning solid rocket propellant grainhaving an active solids content of from 85 to 98 percent by volumecharacterized by a series of intersecting slits located generally alonga line parallel to the axis of the grain and having a width of between0.5 and 5 percent of the diameter of the grain, at least one slitpassing through the central axis of the grain and having other slitscurved in cross-sectional shape located between said central axis andthe outer periphery of the grain.
 2. A stress relieved internallyburning solid rocket propellant grain having an active solids content offrom 85 to 98 percent by volume characterized by at least one slitextending lengthwise along the entire grain and located generally alonga line parallel to the axis of the grain in an area of said grain havingmaximum internal stress and having a width of between 0.5 and 5 percentof the diameter of the grain, said slit having end portions curved incross-section terminating in areas of the grain having minimum internalstress.